As a fluorinated elastomer, a fluorinated elastic copolymer such as a vinylidene fluoride/hexafluoropropylene copolymer, a tetrafluoroethylene/propylene copolymer or a tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer has been known.
Among them, a tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer is particularly excellent in heat resistance and chemical resistance. Accordingly, various studies have been made with respect to such a tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer (e.g. Patent Document 1).
As a process for producing a fluorinated elastic copolymer, a process is, for example, known wherein tetrafluoroethylene and a perfluoro(alkyl vinyl ether) are subjected to radical copolymerization in the presence of a diiodo compound. In this process, the diiodo compound functions as a chain transfer agent, and a fluorinated elastic copolymer obtained by such a process may be crosslinked by e.g. heating in the presence of a peroxide by utilizing iodine atoms bonded to the molecular terminals (the main chain terminals) thereby to obtain a crosslinked rubber article.
However, in such a fluorinated elastic copolymer, iodine atoms to serve as crosslinking points are bonded to the molecular terminals, whereby the molecular weight is substantially influential over the crosslinkability. For example, if the molecular weight is increased, the crosslinking point density decreases, and the crosslinkability decreases. Therefore, there has been a problem that it is difficult to produce a fluorinated elastic copolymer which has a high molecular weight and is excellent in crosslinkability.
To overcome such a problem, a method has been proposed wherein a monomer having a crosslinkable group is copolymerized to obtain a fluorinated elastic copolymer having a crosslinkable group in its side chain. For example, Patent Document 2 proposes a method wherein I(CH2CF2CF2O)m(C(CF3)CF2O)nCF═CF2 is copolymerized to obtain a fluorinated elastic copolymer having an iodine atom in its side chain. Patent Document 3 proposes a method wherein perfluoro(8-cyano-5-methyl-3,6-dioxa-1-octene) is copolymerized to obtain a perfluoropolymer having a cyano group in its side chain.
However, the fluorinated elastic copolymer obtainable by copolymerizing a monomer having a crosslinkable group has a problem such that the physical properties of the obtainable crosslinked rubber are unstable.